68 A MANUAL OF BACTERIOLOGY 



logarithm of the reciprocal of the H-ion concentration 

 expressed in grams of the substance per litre. By this 

 method, the H-ion concentration of pure water is ex- 

 pressed by P H = 7-0, which is neutrality. Because P H 

 is an invert logarithm, it decreases with increasing H-ion 

 concentration. Acid solutions, therefore, have a P H 

 value less than 7-0, and basic solutions a P H value greater 



N 

 than 7-0. Thus, for hydrochloric acid, P H == 1-0 ; 



N N 



for ~ acetic acid, P H = 2-5 ; for caustic soda, 



P H == 11-3. 



In adjusting the standard reaction of culture media 

 by titration, using phenolphthalein as an indicator, by 

 the method described above, difficulty arises owing to the 

 end point being ill denned, and the P H value is now being 

 used to attain greater precision. Now, it would be out 

 of the question to employ electrolytic methods to estimate 

 the P H value, but fortunately new indicators have been 

 introduced which indicate the P H value -within narrow 

 limits. Mclntosh and Smart recommend for this purpose 

 cresol-red. The most suitable P H value seems to be 

 about 7-6. The procedure for standardisation is as 

 follows : The medium is titrated with phenolphthalein 

 and the reaction adjusted as described above. Two small 

 samples (3 c.c.) of the standardised medium are diluted 

 in test-tubes with an equal volume of distilled water. 

 To the first tube are added 2 drops of 0-5 alcohol solution 

 of phenolphthalein and to the second 2 drops of 0-02 

 per cent, solution of cresol-red (aqueous solution with 

 15 per cent, of alcohol). If the reaction be correct, no 

 colour change should occur in the first tube, while a rose 

 to pinkish colour should appear in the second. If so, 

 the P H value will be over 7-6, but less than 7-9, the point 

 at which phenolphthalein first shows a trace of pink. 



